Re: double de-clutching

[four.rings@MCIONE.com]

Hello, Club.

A number of folks on the list have asked me to post on the subject.
I am glad that there are a lot of people who want to drive fast and safe
at the same time thus investing their time in getting to know their
car's mechanics and the proper driving techniques.
(Although one anonymous fella yelled at me in CAPS coz evidently he's
new to the netiquette while also thinking that nothing new could exist
beyond of what he already knew :-) But who cares.)

Attached please find two .txt files which were written by me a while ago
as answers to personal enquiries of the folks on A4.org.

Those who know the techtonique can hit the delete key, the stuff is
pretty well known. Since I don't want to bore those of you who are not
interested in the subject, I am sending these files as attachments.

Regards,
************************************************************
Igor Kessel
'89 200TQ -- 18psi (TAP)
'98 A4TQ -- nothing to declare
Philadelphia, PA
USA
http://www.geocities.com/MotorCity/Garage/8949/homepage.html
************************************************************
Driving with a manual transmission.

I was born in Europe and happened across an automatic car for the first time in
my life only when I came to the States 9 years ago, at the age of 29, i.e. I
belong to the old school which learned to drive on cars with the 3-speed,
non-synchronised gearboxes with strait-cut gears. Which means that unless you
master the art of precise rev-matching (double clutching), you not only ain't
gonna get the gears engaged, but also risk damaging the gearbox by grinding the
gears.

RE: double clutching.
In English this term unfortunately describes the process of rev matching in both
the upshifting and the downshifting.
I'd like to distinguish between them. Let's use two different terms:

"Double clutching" on the upshift: 
lift off => depress the clutch => switch into neutral => release the clutch =>
depress the clutch => switch into a higher gear =>  release the clutch => step
on the gas.

"Double declutching" on the downshift:
lift off => depress the clutch => switch into neutral => release the clutch =>
[blip the throttle, increasing the RPMs in order to match the higher RPMs
of the secondary shaft in the gearbox needed for the bigger DIA lower gear] =>
depress the clutch => switch into a lower gear =>  release the clutch => step on
the gas.
Blipping the throttle to the precisely matched RPMs is a no doubt acquired
skill, but if done properly it affords a silky smooth downshift. 
One might argue that this technique is not needed on a modern synchronised
street car in everyday driving. A yes and a no.

Case in point: you are in the 4th doing 70mph on ice or on packed snow. Suddenly
you need to stop the car in emergency (a fruitcake in an automatic SUV thinks
that she/he's invincible and plants her/himself solid into a snow bank, blocking
the road with her/his ugly bath tub on wheels. I see this happen every winter).
If you hit the brakes you're history. You need to drop 2 gears_and_then_brake
with the engine. The trouble is, @ 70mph the gearbox ain't gonna let you engage
the 2nd no matter how hard you'd be pulling that stick. OTOH, you can do it
easily if you know how to double declutch properly. This technique had saved my
butt more times than I care to admit.

I habitually double declutch at all times in any weather and although I would be
first to admit that it is not always necessary, this habit of mine is probably
the only reason why I have never had a single gearbox breakdown on any of my
cars. Ever.

One more tip. Always depress the clutch fully to the floor, enabling a
complete disengagement. Saves your disk/basket.

RE: heel-and-toeing.
It is a very good practice indeed and really helps on a turbocharged car, where
you would otherwise come out of the corner off boost. I used it a lot on the
track up until I installed a blow-by valve on my 200. Now the turbo is just
freewheeling under the lift-off and the turbine does not lose it's momentum.

Regards,
Igor Kessel
'89 200TQ -- 18psi (TAP)
'98 A4TQ -- nothing to declare 
Why double declutch?

It makes the downshifting possible if you're travelling at a high
speed and need to downshift from 5th to 2nd and then to 1st in order to stop
fast and prevent an accident on a sleek road or if you are having a brakes
failure.
Yes, you may lose your tranny by doing this, but I'd rather bust a
couple of gears than kill either myself or someone else.

Greatly simplified, in any gearbox there is a primary shaft, which is connected
to the engine, and a secondary shaft, connected to the differential (and
ultimately to the driven wheels). There's also a set of gears of different
diametres, interconnecting the shafts.

For our purposes let's consider just the 2nd and the 3rd of the 016 gearbox on
my 200TQ. The 2nd has the gear ratio of 34:16=2.125. The 3rd has the gear ratio
of 35:24=1.458.
Remember from Physics (Mechanics, Kinematic Motion) that if the angular
velocities (rad/sec) of two different DIA gears are the same, the tangential
vector of linear velocity (m/s) on the teeth of a larger gear with the bigger
circumference is higher than that of a smaller gear with the smaller
circumference.
Alternatively if you want to equalise their linear velocities  (as to prevent
the gears from grinding at the moment of engagement), you need to change the
ratio of their angular velocities as a reciprocal of their DIA over pi taken to
one another.
Suppose you are driving in 3rd  @ 2000 RPMs.
Now you're depressing the clutch and switching into neutral hence disengaging
the primary shaft from the secondary one. The secondary one continues to turn @
2000 RPMs coz it is connected to the diff and the wheels and the car is still
traveling at the same speed.
Now you're trying to engage the 2nd and in order to acheve the same linear
velocity on the lower gear the primary shaft now has to rotate with the higher
angular velocity by 2.125/1.458=1.457, i.e. 2915 RPMs. 

This means that when you downshift in order to keep the same vehicle velocity
you need to somehow increase the RPMs on the primary shaft to 2915 while in
neutral. That's what you do by blipping the throttle inbetween the downshifts.

With a bit of practice you should be able to do the double declutching
downshifts in under 2 seconds.

Using the same considerations it is easy to show that for the rev matching on
the upshift you need to decrease the angular velocity of the secondary shaft.
That's a lot easier coz all you need to do is make a short pause (by going
through the neutral _without_ blipping the throttle and just let the RPMs drop
naturally due to engine's internal friction).

Regards,
Igor Kessel
'89 200TQ -- 18psi (TAP)
'98 A4TQ -- nothing to declare